The present invention relates generally to communication receivers and, more specifically, to soft bit demapping in orthogonal frequency division multiplexing (OFDM) receivers.
The following disclosure will be described for a digital video broadcasting (DVB) receiver for digital terrestrial television (DTV). The concepts are equally applicable to any other channels of transmission of DTV receivers and to other receivers or standards using orthogonal frequency division multiplexing (OFDM). These may include but not be limited to wireless standards worldwide, such as wireless LAN 802.11a and g, HIPERLAN/2, Digital Audio Broadcasting (DAB), Digital Video Broadcasting Terrestrial (DVB-T), Digital Video Broadcasting for handheld (DVB-H), 802.16 Broadband Wireless Access, etc. The European terrestrial DTV standard DVB-T (ETS 300 744) is based on COFDM technologies to combat multipath fading. See ETSI EN 300 744 V.1.4.1 “Digital Video Broadcasting (DVB): Framing Structures, Channel Coding, and Modulation for Digital Terrestrial Television.”
FIG. 1 shows a block diagram for a typical DVBT receiver. The digital signal processing for a DVBT receiver can be partitioned into three portions. The first portion 10 includes an RF front end 12, and A/D converter 14, an OFDM demodulator 16, a demodulation 18 and a pilot and TPS decoder 19. This receiver front-end signal processing portion performs receiver training, including various synchronization and channel estimation and OFDM demodulation. The demodulation portion includes a QAM demapper. The second portion 20 is the DVBT receiver back-end signal processing block. It performs DVBT inner channel decoding using inner-deinterleaver 21 and Viterbi decoder 22 and outer channel decoding using outer-deinterleaver 24, RS decoder 26 and energy disperse removal 28. The third portion 30 is a MPEG Decoder. An example is shown in U.S. Pat. No. 7,123,669.
A DVB OFDM transmitter modulates all the data-bearing subcarriers in one OFDM symbol by either QPSK, 16-QAM, 64-QAM, non-uniform 16-QAM and 64-QAM constellations. FIGS. 2A-2C shows the QPSK, uniform 16-QAM and 64-QAM constellations, respectively. In an OFDM receiver as shown in FIG. 1, the data-bearing subcarriers will first go through channel correction, QAM demapping, inner-deinterleaving before entering the Viterbi decoder. Soft QAM demapping is able to provide Viterbi decoder with soft input bits that will enable Viterbi decoder to perform significantly better than with hard bit input. However, the complexity of soft demapping operation for higher order constellation such as 64-QAM is extremely significant. The complexity also grows proportional to the number of soft bits required by Viterbi decoder. Another example is shown in U.S. Pat. No. 6,687,315.
The present method of demapping is in a receiver wherein the input signal is a) demodulated into I real and Q imaginary data pairs which was mapped using a constellation having M bits for the I and Q data pairs, b) demapped, c) deinterleaved and d) decoded. The method of demapping includes deriving M intermediate soft bit values yj (j-0˜M-1) for the I and Q data pairs as a function of the spacing in the constellation; and limiting the range of the M values yj. A look-up table index is derived for each of the limited M values yj. A look-up table, having 2N+1 entries for supporting up to N soft bits, is indexed using the derived indices; and K soft bits (K<=N) for each data bit of the I and Q data pairs are outputted.
The range is limited to ±1. The indices are derived by 2K−(2K−1)*yj. The look-up table includes only soft bits.
These and other aspects of the present disclosure will become apparent from the following detailed description of the disclosure, when considered in conjunction with accompanying drawings.